In recent times, the use of light and more specifically laser light to noninvasively peer inside the body to reveal the interior structure has been investigated. This technique is called optical imaging. Optical imaging and spectroscopy are key components of optical tomography. Rapid progress over the past decade has brought optical computed tomography to the brink of clinical usefulness.
In optical tomography, the process of acquiring the data that will ultimately be used for image reconstruction is the first important step. Light photon propagation is not straight-line and techniques to produce cross-section images are mathematically intensive. To achieve adequate spatial resolution, multiple sensors are employed to measure photon flux density at small patches on the surface of the scanned object. The volume of an average female breast results in the requirement that data must be acquired from a large number of patches. The photon beam attenuation induced by breast tissue reduces the available photon flux to an extremely low level and requires sophisticated techniques to capture the low level signals.
U.S. Pat. No. 5,692,511 discloses such a laser imaging apparatus, This apparatus supports a patient in a face-down, prone position on a horizontal surface with a breast vertically pendant through an opening in a table surface. The patient's breast is pendant within a scanning chamber surrounded by an array of detectors, which revolve around the centerline of the scanning chamber. The array of detectors forms a portion of a circle and the scanning chamber and the opening or aperture in the tabletop are therefore circular. Provision is made to accommodate breasts of differing sizes via interchangeable breast centering rings, which provide circular openings or apertures of differing diameters, all centered on the centerline of the scanning chamber.
In such a computed-tomography geometry, it is required that the rotational centerline of the scanning mechanism pass through the object being scanned. Otherwise the laser beam does not impinge upon the object, and no optical transmission data can be obtained. While this constraint is easily met when the scanner is high in the breast, near the chest wall, the breast will likely move off the rotational centerline, as the scan progresses down the breast toward the nipple. Breasts are generally not conical in shape, typically being quite asymmetric from top to bottom, and somewhat asymmetric from left to right. Typically, even with a prone patient, the breast extends further above the nipple than below. The sagging caused by gravity is permanent, even in the prone position.